1. Field of the Invention
The present invention relates to a surface information measuring apparatus and a surface information measuring method for measuring shape information or various physical information (for example, dielectric constant, magnetized state, permeability, viscoelasticity and friction coefficient) of a surface of a sample. As specific surface information measuring apparatus, there are a scanning probe microscope, a surface roughness meter, a hardness meter and an electrochemical microscope.
2. Background Art
Heretofore, a probe microscope has been mainly used in observation of a surface of a sample by atomic level resolution and in recent years, the probe microscope is started to be used also in an industrial field of an optical disk or a semiconductor. Particularly, in the industrial field, there is requested a length measuring function of measurement of a stepped difference, measurement of a pitch or measurement of an angle and in order to meet the request, for example, there are carried out various improvements of using a hard material having an excellent linearity for a Z element, or improving linearity by integrating sensors for respective XYZ axes and feeding back sensor signals.
Here, although various ones of the above-described probe microscope have been provided, as one of them, there is known a probe microscope for moving a probe in XYZ directions by small amounts by a tube type piezoelectric scanner.
As show by, for example, FIG. 12, the probe microscope 60 is provided with a tube type piezoelectric scanner 63 comprising an XY scanning tube actuator 61, and a Z driving tube actuator 62 connected to a front end of an XY scanning tube actuator 61, and a cantilever 64 fixed to a front end of the Z driving tube actuator 62 and having a stylus 64a at a front end thereof.
The XY scanning tube actuator 61 is provided with a tube type piezoelectric member 65, an inner side electrode 66 arranged on an inner side of the tube type piezoelectric member 65 and an outer side electrode 67 arranged on an outer side of the tube type piezoelectric member 65. The outer side electrode 67 is divided in four uniformly in a peripheral direction, and twos of the electrodes opposed to each other constitute an X scanning electrode and a Y scanning electrode.
Further, the Z driving tube actuator 62 is provided with a tube type piezoelectric member, not illustrated, similar to the XY scanning tube actuator 61, an inner side electrode, not illustrated, arranged on an inner side of the tube type piezoelectric member, and a Z driving electrode 68 arranged on an outer side of the tube type piezoelectric member. The inner side electrode of the Z driving tube actuator 62 and the inner side electrode 66 of the XY scanning tube actuator 61 are electrically connected. That is, the two inner side electrodes constitute a common electrode.
Further, the cantilever 64 is fixed to a front end of the Z driving electrode 68.
An explanation will be given of a case of moving the stylus 64a in XYZ directions by the tube type piezoelectric scanner 63 in the probe microscope 60 constituted in this way. First, in the case of moving the stylus in X direction, the two X scanning electrodes 67 opposed to each other are applied with voltages polarities of which differ from each other. Thereby, the tube type piezoelectric member 65 is bent and a total of the XY scanning tube actuator 61 is bent. The bending is transmitted to the Z driving tube actuator 62 provided at the front end of the XY scanning tube actuator 61 and therefore, as a result, the stylus 64a at the front end of the cantilever 64 can be moved in X direction.
Further, similarly, by applying the two Y scanning electrodes 67 opposed to each other with voltages polarities of which differ from each other, the stylus 64a can be moved in Y direction. In this way, by respectively applying voltages simultaneously to the X scanning electrodes 67 and the Y scanning electrodes 67, the stylus 64a can arbitrarily be scanned in XY directions.
Further, by applying voltages to the common electrode and the Z driving electrode 68, the piezoelectric member of the Z driving tube actuator 68 is elongated and contracted in Z direction. Thereby, the stylus 64a can be moved in Z direction.
In this way, by arbitrarily moving the stylus 64a respectively in XYZ directions by the tube type piezoelectric scanner 63 comprising the XY scanning tube actuator. 61 and the Z driving tube actuator 62, shape information of a surface of a sample and various physical information of the sample (for example, dielectric constant, magnetized state, permeability, viscoelasticity and friction coefficient) can be measured.
Here, measurement of angle constituting one of length measurements explained initially measures, for example, an angle of a side wall of a recessed and projected shape of an IC pattern. Particularly, with regard to an angle measuring function, linearity of Z effects a governing influence. The linearity of Z is an index of how much cross talk operation in XY directions is produced in an elongating and contracting operation in Z direction in the above-described Z driving tube actuator 62.
Particularly, the piezoelectric member of the tube type is a thin-walled ceramic circular cylinder (comprising, for example, a material of lead zirconate titanate (abbreviated as PZT)) having a thickness of about 1 mm or smaller, and owing to the thin wall, it is difficult to control accuracy of the wall thickness. Further, since the linearity of Z depends on the wall thickness, it is difficult to promote the angle measuring function by the piezoelectric member of the tube type. Further, the larger the angle to be measured, the larger the influence effected on a result of measuring the angle by an error in the linearity of Z. Particularly, in recent years, there is increased needs of measuring an angle of a side wall proximate to be vertical such as of STI (shallow trench isolation), however, as described above, the Z linearity of the tube type piezoelectric member in a current state is insufficient and an improvement thereof has been desired.
A further specific explanation will be given of the Z linearity. The Z linearity is determined by machining accuracy of coaxiality, or circularity or a nonuniformity in polarization of the tube type piezoelectric member.
Here, an explanation will be given by taking an example of a Z driving tube actuator 70 which is eccentric (a center position of an inner diameter and a center position of an outer diameter are shifted from each other) in reference to FIG. 13. Normally, the tube type piezoelectric member 71 is fabricated by boring to penetrate a center of a piezoelectric member in a circular cylinder type by machining. Therefore, a center position of a hole is shifted by a deficiency in an accuracy in machining and as shown by FIG. 13, there is brought about a state of being eccentric by producing a deviation in the thickness of the piezoelectric member 71 (a state in which a thickness d1 on one side is thinner than a thickness d2 on other side).
In this way, when voltages are applied to the eccentric piezoelectric member 71, an electric field is intensified on a thin side of the piezoelectric member 71 (d1 side) and therefore, an elongating and contracting amount is large, on the other hand, the electric field is weakened on a thick side of the piezoelectric member 71 (d2 side) and therefore, the elongating and contracting amount is reduced. As a result, as shown by FIG. 13, the piezoelectric member 71 is not elongated and contracted straight in Z direction and is bent in XY directions, that is, cross talk in XY directions is produced. That is, in accordance of displacing the probe by ΔDz, also a displacement ΔDx in X direction is produced. The error is increased particularly when a ratio of the displacements ΔDz to ΔDx (ΔDx/ΔDz) is large.
In this way, when the piezoelectric member becomes eccentric, the linearity in Z direction is deteriorated. As a result, there is a drawback that the length measurement such as of measurement of an angle cannot be carried out with high accuracy. Further, similarly, also a circularity of the piezoelectric member and the nonuniformity in polarization similarly cause to deteriorate the linearity in Z direction constituting a cause of being unable to carry out the length measurement with high accuracy.
However, since the piezoelectric member is a thin-walled ceramic circular cylinder of substantially 1 mm or smaller, there is a limit in promoting machining accuracy and it is difficult to finish the coaxiaility and the circularity with high accuracy simply by machining. Further, it is similarly difficult to control the nonuniformity in polarity with high accuracy. Further, there is also a drawback of increasing cost for fabrication and taking time and labor.
On the other hand, there is known a parallel spring type scanner excellent in linearity and giving importance on length measuring performance in which the above-described cross talk in XY directions of the tube type piezoelectric member does not pose a problem. As shown by, for example, FIG. 14, the parallel spring type scanner 80 is provided with a metal plate 81 in a shape of a frame, a movable portion 83 movable in one direction attached to the metal plate 81 by interposing a parallel spring portion 82 formed by cutting a groove by a wire cut electric discharge machine, and a stack type piezoelectric element 84 for moving the movable portion 83 in one direction relative to the metal plate 81.
According to the parallel spring type scanner 80, the movable portion 83 can be moved straight in one direction by applying voltages to the stack type piezoelectric element 84 and therefore, the parallel spring scanner 80 is excellent in linearity. Further, by surrounding the periphery of the metal plate 81 by a second metal plate, not illustrated, in a shape of a frame and providing the parallel spring portion 82 and the limited type piezoelectric element 84 between the second metal plate and the metal plate 81, the second metal plate can be moved in other direction orthogonal to the one direction. Further, the movable portion 83 can be moved straight in three directions of XYZ by constituting the movable portion 83 by further combining with the parallel spring portion 82 and the stack type piezoelectric element 84 to be able to move the second metal plate in a direction orthogonal to the one direction and the other direction.
However, the parallel spring type scanner 80 is provided with a drawback that in comparison with the above-described tube type piezoelectric scanner 63, although the parallel spring type scanner 80 is excellent in linearity, the parallel spring type scanner 80 is large-sized to need an installing space and further, cost thereof is increased and a resonance frequency is low.
In this respect, the tube type piezoelectric scanner is preferably used. Hence, there is known a probe position correcting method for correcting cross talk in XY directions by utilizing the tube type piezoelectric scanner (refer to, for example, Patent Reference 1).
The probe position correcting method is provided with a stage of measuring an amount of moving a probe in XY face in accordance with movement of the probe in Z direction from an observed image and a stage of correcting the probe position based on the moving amount. Thereby, the error caused by moving the probe in XY face in accordance with movement of the probe in Z direction can be corrected and the observed image of a sample with high accuracy can be provided.
[Patent Reference 1] JP-A-2004-4026
However, according to the method of the related art, the following problem remains to be posed.
That is, the probe position correcting method described in Patent Reference 1 is on the promise that the Z driving tube actuator is not elongated or contracted straight and correction in XY directions is simply carried out based on the provided observed image. That is, the observed image observed by the probe finally is the observed image cross talk in XY directions of which is corrected, however, the Z driving tube actuator per se is not elongated or contracted straight. Therefore, the Z driving tube actuator is inclined in XY directions or is provided with a kinetic energy in transverse direction (XY directions) and therefore, there is a concern that noise is caused and thereby accurate measurement of the sample cannot be carried out.
Further, it is necessary to output a corrected voltage in accordance with a moving amount in Z direction to XY directions and therefore, there poses a problem that a circuit is complicated.
Further, it is also difficult to deal with cross talk in XY directions by utilizing a software. That is, although movement of the piezoelectric member in XY directions can be processed to linearize by a software, with regard to movement in Z direction, modeling of motion is impossible in fact. This is because that the piezoelectric member of PZT or the like is provided with a hysteresis and therefore, there are two heights for an applied voltage, further, an amount of the hysteresis is changed by the moving amount or an offset amount. Therefore, it is difficult to deal therewith by utilizing a software. Therefore, the probe position correcting method described in Patent Reference 1 is obliged to be depended.